IS ULTRAFILTRATION BETTER THAN MICROFILTRATION AS PRETREATMENT FOR REVERSE OSMOSIS? – PILOT SCALE RESULTS

Granular-media filtered secondary effluent from a full-scale plant was subsequently treated at pilot-plant scale by combinations of low- and high pressure membranes. The feed water was split between microfiltration (MF) and ultrafiltration (UF) modules; ferric chloride (4 mg/L) was added to the UF feedwater. Filtrate from each of these became the feed water to three different types of high-pressure membranes operating in parallel, two reverse osmosis (RO) units and one nanofiltration (NF) unit; no chemicals (e.g., chlorine) were added ahead of the high-pressure membranes to control biofouling. Both the low- and high pressure membrane systems were operated at constant flux such that fouling was measured by an increase in transmembrane pressure (TMP). Rejection efficiency was measured by total organic carbon (TOC), selected inorganic parameters, particle counts, turbidity and a virus challenge study. Specific flux was higher and the average TMP was lower for the UF system than the MF system. The fouling rate of two of the three types of high-pressure membranes, as measured by the initial specific flux after cleaning and by the subsequent decline in specific flux, was lower for pretreatment by UF than by MF. Membrane polymer chemistry could possibly explain why the choice of pretreatment did not affect the fouling rate of one of three high-pressure membranes. The choice of membrane pretreatment did not affect rejection efficiency of any of the three high-pressure membranes.